1. Field of the Invention
The present invention relates to a noise cancel circuit which removes noise using an interpolation technique.
2. Description of the Background Art
Generally, when a pulse noise generated by a vehicle engine overlaps with the audio band, a car radio receiver will produce an audible noise unpleasant to listeners. To prevent this, radio receivers usually include a noise cancel circuit. Such a noise cancel circuit removes a pulse noise from an audio signal by performing pre-hold processing on the signal during noise generation. A conventional noise cancel circuit is shown in FIG. 3.
In FIG. 3, an input signal including noise is supplied via the input terminal 31 to the delay circuit 32 and HPF 33. After passing through the HPF 33, if noise exceeding a certain predetermined level is detected by the noise detection circuit 34, the noise detection circuit 34 produces an output pulse. The output pulse is applied to the gate generation circuit 35, which generates a gate signal having a predetermined width.
In response to the gate signal, the switch 36 is turned off. As a result, application of a pilot cancel signal to the subtraction circuit 37 is stopped, thereby allowing the output signal from the delay circuit 32 to be supplied to both input terminals of the subtraction circuit 37. The output AC signal from the delay circuit 32 is canceled by the subtraction circuit, and the DC voltage between the two ends of the capacitor 38 results at the output end of the subtraction circuit 38. The delay time of the delay circuit 32 equals the time required for the processing from the HPF 33 to the switch 36. In this way, noise within the input signal is removed. Subsequently, when the gate signal generation is stopped, the switch 36 is turned on, and the pilot cancel signal is applied to the subtraction circuit 37. The pilot signal component within the input signal is then canceled by the subtraction circuit 37.
However, as the frequency band of noise such as pulse noise is often within the audio frequency band, the noise removal by pre-hold processing removes not only noise, but also sound. While noise removal by pre-hold processing may produce an output that sounds improved to the ear, the pre-hold processing also actually decreases the distortion factor, thereby newly introducing deterioration of sound quality.